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Cholesterol, atheroma and what a risk factor means questions

Cholesterol as a sterol and what it is needed for; lipoproteins as vehicles, with chylomicrons, VLDL, LDL and HDL distinguished by density and cargo; LDL receptors and familial hypercholesterolaemia; the formation of an atheroma from endothelial damage through foam cells to a fibrous plaque; thrombosis, myocardial infarction and aneurysm; risk factors, absolute risk scores and what the term licenses; and how the evidence on LDL, HDL and dietary fat was gathered.

5 original questions · 20 marks · the cholesterol, atheroma and what a risk factor means notes · Nutrition, digestion and health evidence

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• 5 marks · Describe

  Describe the formation of an atheroma, in order, from the first damage to the artery wall through to a fibrous plaque.

  Mark scheme

  1. B1 the endothelium lining the vessel is damaged by mechanical stress from high blood pressure, particularly where arteries branch, by chemicals in tobacco smoke, by high blood glucose or by a high LDL concentration
  2. B1 LDL particles pass into the wall beneath the endothelium, are retained there and are chemically modified by oxidation, so they are treated as damage rather than as cargo
  3. B1 monocytes adhere to the activated endothelium and migrate into the wall, where they become macrophages
  4. B1 the macrophages take up modified LDL through receptors that are not regulated by how much cholesterol the cell already holds, so they keep eating, fill with lipid and become foam cells, and a collection of foam cells is a fatty streak
  5. B1 smooth muscle cells migrate from the deeper layers, multiply and deposit collagen over the accumulating lipid, forming a fibrous cap, so the plaque bulges into the lumen and narrows it

• 4 marks · Explain

  Explain how the rupture of an atheroma in a coronary artery leads to the death of cardiac muscle.

  Mark scheme

  1. B1 the fibrous cap tears, so collagen beneath it is exposed to the blood and platelets adhere and release thromboplastin
  2. B1 thromboplastin with calcium ions catalyses the conversion of prothrombin to thrombin, and thrombin catalyses the conversion of soluble fibrinogen into insoluble fibrin, which forms a mesh trapping cells
  3. B1 that clot can occlude the artery within minutes, so blood no longer reaches the muscle downstream of the blockage
  4. B1 the muscle is therefore deprived of oxygen and cannot respire aerobically, so its supply of ATP fails and the cells die, which is a myocardial infarction

• 4 marks · Compare

  Compare LDL particles with HDL particles, referring to their composition, what each carries and where each takes it.

  Mark scheme

  1. B1 both are lipoproteins, with a core of cholesteryl esters and triglyceride wrapped in a single layer of phospholipid carrying apoproteins, and the cholesterol inside each is chemically the same molecule
  2. B1 an LDL particle is larger and cholesterol-rich with relatively little protein, whereas an HDL particle is smaller and protein-rich, and since protein is dense that is why HDL is the denser class
  3. B1 LDL delivers cholesterol to any cell carrying LDL receptors, which take the whole particle in by receptor-mediated endocytosis, whereas HDL accepts cholesterol from cell membranes, including from cells inside artery walls
  4. B1 HDL carries what it collects back to the liver, which can excrete it in bile, so the two classes of particle run in opposite directions

• 4 marks · Explain

  Explain why the evidence that a high LDL concentration causes atherosclerosis is regarded as much stronger than the evidence that a high HDL concentration protects against it.

• 3 marks · State

  State what kind of lipid cholesterol is, and state what the body needs it for.

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